Risers are widely used with subsea oil wells. A riser is a tubular attached at one end to a wellhead on the seabed and at the other end to a floating rig or a boat on the surface. The riser passes through a hole in the rig floor or the deck of the boat, and is topped by a riser access device, which may incorporate valves, to allow fluids to enter or leave the well, or plugs or the like.
The riser must be maintained in tension to prevent it from collapsing, and tension is applied to the riser by tensioners which extend from the vessel and attach to the riser at a location beneath the vessel deck or floor. The tensioners are generally cables which pay out and reel in as the vessel moves due to the heave of the sea, maintaining a constant tension on the riser, and maintaining the vertical position of the access device fixed relative to the seabed. As the access device is fixed relative to the seabed, and to compensate for the rise and fall of the vessel, the access device is located at a considerable height above the vessel, to prevent the riser access device impacting on the floor or deck of the vessel.
However, when is necessary for an operator access to the riser, it is undesirable for the access device to be moving relative to the vessel floor or deck; instead it is preferable for the access device to remain stationary relative to the vessel. To make this possible, a slip joint is provided between the access device and the point at which the tensioners attach to the riser. The slip joint accommodates the heave of the sea permitting the access device to be lowered to the deck or floor. To maintain the riser in tension when access is not required, a compensator applies a lifting force to the access device sufficient to extend the slip joint to the maximum extent of its travel.
The compensator can apply the lifting force to the access device through a lifting frame which may be provided above the access device to permit equipment such as injector valves to be attached to the access device or to allow tool strings to be lowered through the access device into the riser.
Conventional riser systems have a number of drawbacks. For example, the slip joint can be unreliable and can only be safely utilized when depressurised as they are subject to a pressure end load effect. When the slip joint is depressurised the surface isolation valves cannot be utilised and if there is any leakage past the primary downhole well isolation device both personnel on the vessel and the vessel itself can be exposed to hydrocarbons from the well and the associated consequential risk.
Furthermore, in rough seas or in dynamic positioning mode it can be difficult to maintain the position of the vessel directly over the subsea wellhead. Deviation from this position applies a bending moment to the riser, which can result, in some cases, in the vessel having to detach from the wellhead to avoid catastrophic failure. To accommodate the movement of the vessel, the surface equipment has to placed a considerable height above the vessel floor or deck to avoid it impacting with the vessel floor as the vessel moves off location due to the increased distance between the vessel and the well.
There are also safety issues associated with accessing the riser from above the flowhead. The elevated positioning of the flowhead and the lifting frame requires high level working on a platform which is moving in relation to the vessel floor or deck to introduce tools or equipment into the riser.
A further area of concern is during operation, the compensator system, which applies a tension to the riser through the surface equipment, such as the lifting frame, has been known to over-tension the riser causing the riser to fail with catastrophic consequences.
An object of at least one embodiment of the present invention is to obviate or mitigate at least one of the disadvantages of the aforementioned well intervention systems.